The identification of small organic molecules that affect specific biological functions is an endeavor that impacts both biology and medicine. Such molecules are useful as therapeutic agents and as probes of biological function. Such small molecules have been useful at elucidating signal transduction pathways by acting as chemical protein knockouts, thereby causing a loss of protein function. (Schreiber et al, J. Am. Chem. Soc, 1990, 112, 5583; Mitchison, Chem. and Biol., 1994, I5 3) Additionally, due to the interaction of these small molecules with particular biological targets and their ability to affect specific biological function (e.g. gene transcription), they may also serve as candidates for the development of new therapeutics.
One biological target of recent interest is histone deacetylase (HDAC) (see, for example, a discussion of the use of inhibitors of histone deacetylases for the treatment of cancer: Marks et al. Nature Reviews Cancer 2001, 7,194; Johnstone et al. Nature Reviews Drug Discovery 2002, 287). Post-translational modification of proteins through acetylation and deacetylation of lysine residues plays a critical role in regulating their cellular functions. HDACs are zinc hydrolases that modulate gene expression through deacetylation of the N-acetyl-lysine residues of histone proteins and other transcriptional regulators (Hassig et al Curr. Opin. Chem. Biol. 1997, 1, 300-308). HDACs participate in cellular pathways that control cell shape and differentiation, and an HDAC inhibitor has been shown effective in treating an otherwise recalcitrant cancer (Warrell et al J. Natl. Cancer Inst. 1998, 90, 1621-1625). At this time, eleven human HDACs, which use Zn as a cofactor, have been identified (Taunton et al. Science 1996, 272, 408-411; Yang et al. J. Biol. Chem. 1997, 272, 28001-28007. Grozinger et al. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 4868-4873; Kao et al. Genes Dev. 2000, 14, 55-66. Hu et al J. Biol. Chem. 2000, 275, 15254-15264; Zhou et al. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 10572-10577; Venter et al. Science 2001, 291, 1304-1351) these members fall into three classes (class I, II, and IV). An additional seven HDACs have been identified which use NAD as a cofactor. To date, no small molecules are known that selectively target any particular class or individual members of this family (for example ortholog-selective HDAC inhibitors have been reported: (a) Meinke et al. J. Med. Chem. 2000, 14, 4919-4922; (b) Meinke, et al Curr. Med. Chem. 2001, 8, 211-235). There remains a need for preparing structurally diverse HDAC and tubulin deacetylase (TDAC) inhibitors particularly ones that are potent and/or selective inhibitors of particular classes of HDACs or TDACs and individual HDACs and TDACs.
Recently, a cytoplasmic histone deacetylase protein, HDAC6, was identified as necessary for aggresome formation and for survival of cells following ubiquitinated misfolded protein stress. The aggresome is an integral component of survival in cancer cells. The mechanism of HDAC6-mediated aggresome formation is a consequence of the catalytic activity of the carboxy-terminal deacetylase domain, targeting an uncharacterized non-histone target. The present invention also provides small molecule inhibitors of HDAC6. In certain embodiments, these new compounds are potent and selective inhibitors of HDAC6.
The aggresome was first described in 1998, when it was reported that there was an appearance of microtubule-associated perinuclear inclusion bodies in cells over-expressing the pathologic AF508 allele of the cystic fibrosis transmembrane conductance receptor (CFTR). Subsequent reports identified a pathologic appearance of the aggresome with over-expressed presenilin-1 (Johnston J A, et al. J Cell Biol. 1998; 143:1883-1898), parkin (Junn E, et al. J Biol Chem. 2002; 277: 47870-47877), peripheral myelin protein PMP22 (Notterpek L, et al. Neurobiol Dis. 1999; 6: 450-460), influenza virus nucleoprotein (Anton L C, et al. J Cell Biol. 1999; 146:113-124), a chimera of GFP and the membrane transport protein pi 15 (Garcia-Mata R, et al. J Cell Biol. 1999; 146: 1239-1254) and notably amyloidogenic light chains (Dui J L, et al. J Cell Biol. 2001; 152:705-716). Model systems have been established to study ubiquitinated (AF508 CFTR) (Johnston J A, et al. J Cell Biol. 1998; 143:1883-1898) and non-ubiquitinated (GFP-250) (Garcia-Mata R, et al. J Cell Biol. 1999; 146:1239-1254) protein aggregate transport to the aggresome. Secretory, mutated, and wild-type proteins may assume unstable kinetic intermediates resulting in stable aggregates incapable of degradation through the narrow channel of the 26S proteasome. These complexes undergo active, retrograde transport by dynein to the pericentriolar aggresome, mediated in part by a cytoplasmic histone deacetylase, HDAC6 (Kawaguchi Y, et al. Cell. 2003; 1 15:727-738).
Histone deacetylases are a family of at least 11 zinc-binding hydrolases, which catalyze the deacetylation of lysine residues on histone proteins. HDAC inhibition results in hyperacetylation of chromatin, alterations in transcription, growth arrest, and apoptosis in cancer cell lines. Early phase clinical trials with available nonselective HDAC inhibitors demonstrate responses in hematologic malignancies including multiple myeloma, although with significant toxicity. Of note, in vitro synergy of conventional chemotherapy agents (such as melphalan) with bortezomib has been reported in myeloma cell lines, though dual proteasome-aggresome inhibition was not proposed. Until recently selective HDAC inhibitors have not been realized.
HDAC6 is required for aggresome formation with ubiquitinated protein stress and is essential for cellular viability in this context. HDAC6 is believed to bind ubiquitinated proteins through a zinc finger domain and interacts with the dynein motor complex through another discrete binding motif. HDAC6 possesses two catalytic deacetylase domains. It is not presently known whether the amino-terminal histone deacetylase or the carboxy-terminal tubulin deacetylase (TDAC) domain mediates aggresome formation.
Aberrant protein catabolism is a hallmark of cancer, and is implicated in the stabilization of oncogenic proteins and the degradation of tumor suppressors (Adams J. Nat Rev Cancer. 2004; 4:349-360). Tumor necrosis factor alpha induced activation of nuclear factor kappa B (NFKB) is a relevant example, mediated by NFKB inhibitor beta (IKB) proteolytic degradation in malignant plasma cells. The inhibition of IKB catabolism by proteasome inhibitors explains, in part, the apoptotic growth arrest of treated myeloma cells (Hideshima T, et al. Cancer Res. 2001; 61:3071-3076). Multiple myeloma is an ideal system for studying the mechanisms of protein degradation in cancer. Since William Russell in 1890, cytoplasmic inclusions have been regarded as a defining histological feature of malignant plasma cells. Though the precise composition of Russell bodies is not known, they are regarded as ER-derived vesicles containing aggregates of monotypic immunoglobulins (Kopito R R, Sitia R. EMBO Rep. 2000; 1:225-231) and stain positive for ubiquitin (Manetto V, et al. Am J Pathol. 1989; 134:505-513). Russell bodies have been described with CFTR over-expression in yeast (Sullivan M L, et al. J. Histochem. Cytochem. 2003; 51:545-548), thus raising the suspicion that these structures may be linked to overwhelmed protein catabolism, and potentially the aggresome. The role of the aggresome in cancer remains undefined.
Aberrant histone deacetylase activity has also been linked to various neurological and neurodegenerative disorders, including stroke, Huntington's disease, Amyotrophic Lateral Sclerosis and Alzheimer's disease. HDAC inhibition may induce the expression of anti-mitotic and anti-apoptotic genes, such as p21 and HSP-70, which facilitate survival. HDAC inhibitors can act on other neural cell types in the central nervous system, such as reactive astrocytes and microglia, to reduce inflammation and secondary damage during neuronal injury or disease. HDAC inhibition is a promising therapeutic approach for the treatment of a range of central nervous system disorders (Langley B et al., 2005, Current Drug Targets-—CNS & Neurological Disorders, 4: 41-50).
Histone deacetylase is known to play an essential role in the transcriptional machinery for regulating gene expression, induce histone hyperacetylation and to affect the gene expression. Therefore, it is useful as a therapeutic or prophylactic agent for diseases caused by abnormal gene expression such as inflammatory disorders, diabetes, diabetic complications, homozygous thalassemia, fibrosis, cirrhosis, acute promyelocytic leukaemia (APL), organ transplant rejections, autoimmune diseases, protozoal infections, tumors, etc. Thus, there remains a need for the development of novel inhibitors of histone deacetylases and tubulin histone deacetylases. In particular, inhibitors that are more potent and/or more specific for their particular target than known HDAC and TDAC inhibitors are needed. HDAC inhibitors specific for a certain class or member of the HDAC family would be particularly useful both in the treatment of proliferative diseases and protein deposition disorders and in the study of HDACs, particularly HDAC6. Inhibitors that are specific for HDAC versus TDAC and vice versa are also useful in treating disease and probing biological pathways. The present invention provides novel compounds, pharmaceutical compositions thereof, and methods of using these compounds to treat disorders related to HDAC6 including cancers, inflammatory, autoimmune, neurological and neurodegenerative disorders.